<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
    <script>
      // 20.   用户输入一个数字n，计算1+2+3+4+……n的和。
      var n = 0;
      var num = parseInt(prompt("请输入一个数字。"));
      for (i = 1; i <= num; i++) {
        n += i;
      }
      console.log("总数：" + n);
    </script>
  </head>
  <body></body>
</html>
